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In cell biology, molecular biology and related fields, the word extracellular (or sometimes extracellular space) means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid. The term is used in contrast to intracellular (inside the cell).

The composition of the extracellular space includes metabolites, ions, proteins, and many other substances that might affect cellular function. For example, hormones act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are the digestive enzymes.

The term 'extracellular' is often used in reference to the extracellular fluid (ECF) compartment which composes about 15 litres of an average adult 70 kg human body which is assumed to contain a total of about 50 litres of water (thus, about 30% of the body's water is in the ECF compartment).

The cell membrane (and, in plants and fungi, the cell wall) is the barrier between the two, and chemical composition of intra- and extracellular milieu can be radically different. In most organisms, for example, a Na+/K+-ATPase pump maintains a high concentration of sodium ions outside cells while keeping that of potassium low, leading to chemical excitability. Many cold-tolerant plants force water into the extracellular space when the temperature drops below 0 degrees Celsius, so that when it freezes, it does not lyse the plants' cells. [1]

Two compartments comprise the extracellular space: the vascular space and the interstitial space.[2]

References

  1. ^ Taiz, Lincoln. Plant Physiology, 4th ed. 2006. Sinaeur Associates, Inc.
  2. ^ Fleischhauer J, Lehmann L, Kléber AG (August 1995). "Electrical resistances of interstitial and microvascular space as determinants of the extracellular electrical field and velocity of propagation in ventricular myocardium". Circulation 92 (3): 587–94. PMID 7634473. http://www.circ.ahajournals.org/cgi/content/full/92/3/587. 
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